1. Field of the Invention
The present invention relates to a sheet material feeder provided in image forming apparatuses such as printers, copying machines and facsimiles. More particularly, the present invention relates to a sheet material feeder provided with sheet-material lateral position adjusting means for adjusting a widthwise position of a sheet material (referred to hereinafter as xe2x80x9clateral registrationxe2x80x9d), and an image forming apparatus including the sheet material feeder.
2. Description of the Related Art
Conventional image forming apparatuses such as printers, copying machines and facsimiles sometimes include a sheet material feeder provided with a reversing mechanism for turning a sheet material upside down to print an image on the second or opposite side of the sheet material. By way of example, FIG. 11 shows an overall construction of such a conventional image forming apparatus provided with a sheet material reversing mechanism.
In FIG. 11, a sheet supply cassette N accommodating a number of sheet materials S piled one above another is detachably attached to a bottom portion of an image forming apparatus body M.
Numeral 1 denotes a sheet feed roller, and 2 denotes a transport roller in pressure contact with a pair of rollers 2a, 2b. Numeral 3 denotes a register roller pair, 4 denotes a photoconductive drum, and 5 denotes a transfer roller. Numeral 6 denotes a fusing roller pair, 7 denotes a transport roller pair, and 8 denotes a transport roller capable of rotating forward and backward selectively (referred to as a xe2x80x9creversing rollerxe2x80x9d hereinafter). Numeral 9 denotes a semicircular roller having a semicircular shape in section, and 9a denotes a driven roll against which the semicircular roller 9 is brought in pressure contact. Numeral 10 denotes a transport roller pair, and 11 denotes a transport roller pair with the function of removing a curl from the sheet. Numeral 12 denotes a scanner mechanism, 13 denotes a returning mirror, and 14 denotes a sheet ejection tray. Numeral 15 denotes a flapper for changing the direction of transport of the sheet material S, and 16 denotes a guide in the form of a pair of sheet material guide plates.
A beam of light scanned by a scanner 12 is introduced to the photoconductive drum 4 after being reflected by the returning mirror 13.
In the image forming apparatus shown in FIG. 11, the guide 16 serves as sheet-material lateral position adjusting means for adjusting a position of the sheet material S in the lateral direction.
FIG. 12 shows a detailed construction of a portion of the sheet material lateral position adjusting means including the guide 16. As shown in FIG. 12, the guide 16 comprises left and right lateral register plates (sheet-material lateral position adjusting means) 16a, 16b each being substantially U- or channel-shaped in section. The left and right lateral register plates 16a, 16b are arranged such that their openings face each other with a predetermined spacing between the plates.
One 16b of the lateral register plates is attached for sliding movement on a base plate 16c, and is biased inward by a compression spring 17 interposed between the lateral register plate 16b and a spring seat 16e of the base plate 16c. The lateral register plate 16b engages a stopper 16d of the base plate 16c for restriction of further sliding movement and tends to be held in the engaging condition.
The base plate 16c of the one lateral register plate 16b and the other lateral register plate 16a are provided for movement in directions of arrows B perpendicular to the direction of transport of the sheet material, and are moved by driving means such as cylinders 19a, 19b. 
The cylinders 19a, 19b are controlled by a CPU (control means) 23, which is a control unit for the entirety of the image forming apparatus, through cylinder drives 20a and 20b, respectively.
A semicircular roller 9 is disposed approximately midway between the left and right lateral register plates 16a, 16b in the direction perpendicular to the direction of transport of the sheet material, and rotation of a motor 9b is transmitted to the semicircular roller 9. The motor 9b is controlled by the CPU 22 through a motor driver 21.
Returning to FIG. 11, the sheet material S advanced by the sheet feed roller 1 from the sheet supply cassette N is introduced to a nip between the transport roller 2 and the roller 2a, and is further transported to the register roller pair 3.
Then, the sheet material S is advanced by the register roller pair 3 toward the photoconductive drum 4 in timed relation therewith, and a toner image formed on the photoconductive drum 4 is transferred onto the sheet material S.
Subsequently, the sheet material S is transported to the fusing roller pair 6 for permanent fusing of the toner image. The sheet material S is then fed to the flapper 15 by the transport roller pair 7.
In the case of printing images on both surfaces of the sheet material S, the sheet material S with an image having been printed on a first surface thereof is fed by transport roller pair 7 and guided in a direction of arrow C by the flapper 15 held in a posture indicated by broken lines, and is then introduced to the reversing roller 8.
FIG. 13 shows in detail a manner of turning the sheet material S upside down with the reversing roller 8. The sheet material S having been advanced in the direction C passes a crotch point 18a positioned at a lower end of a guide 18, and a leading end of the sheet material S reaches sensors 23a, 23b which serve as sheet-material detecting means which are disposed downstream of the crotch point 18a. 
Upon the sensors 23a, 23b detecting the leading end of the sheet material S, a reversing roller pair 8a, 8b are rotated in directions of respective solid-line arrows to advance the sheet material S until a tailing end of the sheet material S is detected by the sensors 23a, 23b. 
Detection of the tailing end of the sheet material S indicates that the tailing end of the sheet material S has passed the crotch point 18a, and the reversing roller pair 8a, 8b are now rotated in directions of respective broken-line arrows.
Here, a nip between the reversing roller pair 8a, 8b is oriented in a direction D indicated by a broken-line arrow. Therefore, when the tailing end of the sheet material S has passed the crotch point 18a and the reversing operation is started, a new leading end (the tailing end before the reversing operation) of the sheet material S is forwarded to move in the direction D. The sheet material S advanced in the direction D is introduced to the guide 16.
The CPU 22 shown in FIG. 12 receives detection signals from the sensors 23a, 23b (these being denoted by 23 in FIG. 12), and controls the semicircular roller 9 and the lateral register plates 16a, 16b in accordance with the received detection signals.
The sheet material S is transported in the reversed state by the reversing roller 8, and is further transported by the semicircular roller 9 and the driven roll 9a until the tailing end of the sheet material S passes beyond the reversing roller 8. The sheet material S is then stopped while it locates in channel-shaped grooves of the lateral register plates 16a, 16b. 
At the time when the sheet material S enters the channel-shaped grooves of the lateral register plates 16a, 16b, the lateral register plates 16a, 16b are positioned so as to provide a spacing therebetween which is greater than the width of the sheet material S. Subsequently, the base plate 16c of the one lateral register plate 16b and the lateral register plate 16a are moved in the directions of respective arrows B perpendicular to the direction of transport of the sheet material to reference positions that are different depending on the size of each sheet material, thereby adjusting a lateral registry position of the sheet material S in the lateral register plates 16a, 16b. 
FIG. 14 shows the lateral register plates 16a, 16b with the spacing therebetween greater than the width of the sheet material S; that is, at the time when the sheet material S enters the channel-shaped grooves of the lateral register plates 16a, 16b. If the sheet material S in the guide 16 is skewed as shown in FIG. 14, the skew of the sheet material S is corrected because the sheet material S is pushed by the lateral register plates 16a, 16b which are moved in the directions of respective arrows B.
More specifically, the lateral register plate 16a pushes a lateral edge S1 of the sheet material S to turn the sheet material S in a direction to cancel the skew, and the lateral register plate 16b pushes an opposite lateral edge S2 of the sheet material S to turn the sheet material S in the same direction. As a result, the skew of the sheet material S is eliminated.
When the base plate 16c and the lateral register plate 16a are stopped in the reference positions, the sheet material S is positioned without play between both the lateral register plates 16a, 16b as shown in FIG. 15. At the same time, lateral register plate 16b is urged by the compression spring 17 toward lateral register plate 16a and the sheet S is correctly held in the lateral registry position.
Returning to FIG. 11 and continuing the description, after the lateral registry position of the sheet material S has been adjusted, the sheet material S is introduced to the transport roller pair 10 by the semicircular roller 9 and the driven roll 9a. When the sheet material S is introduced to the transport roller pair 10, the lateral register plates 16a, 16b are moved back to the original positions providing a spacing therebetween greater than the width of the sheet material S.
Then, the sheet material S is introduced to a nip between the transport roller 2 and the roller 2b, and is further introduced to the nip between the transport roller 2 and the roller 2a. After that, the sheet material S is transported to the register roller pair 3 with a second surface facing upward. The sheet material S is forwarded from the register roller pair 3 to the photoconductive drum 4 where an image is printed on the second surface of the sheet material S in the same manner as in the case for the first surface.
The sheet material S having been subjected to printing on the second surface (or the sheet material S on which an image is to be printed on one surface and which has been subjected to printing) is advanced by transport roller pair 7 and guided in a direction of solid-line arrow F by the flapper 15 held in a posture indicated by solid lines in FIG. 11, and is then ejected on the tray 14 by the transport roller pair 11, which also functions to remove a sheet curl.
In the related art described above, however, because a transport path defined by the guide 18 and an opposing guide 24, both serving as sheet material guide means, are curved sharply as shown in FIG. 16, the following drawbacks have occurred.
(1) When the sheet material S is a sheet of thick paper, the sheet material S has a relatively strong stiffness. Therefore, when the sheet material S is stopped in the lateral registry adjustment position, it is urged downward by the crotch point 18a of the guide 18 and a guide surface 16l of the guide 16, and is urged upward by a top 24a of the guide 24. In other words, the sheet material S is brought into a tightly stretching condition supported at three points, i.e., the crotch point 18a, the guide surface 16l and the top 24a (as shown in FIG. 16B).
In the case where the sheet material S is transported to the guide 16 in a state skewed toward the side of the lateral register plate 16b, even with the lateral register plates 16a, 16b operated to move toward the reference positions in the directions of respective arrows B shown in FIG. 14, the sheet material S is not moved by the lateral register plate 16b in the direction of arrow B because of being supported at the three points. The urging force of the compression spring 17 is overwhelmed by the tight stretching of the sheet material S, and the lateral register plate 16b cannot move the sheet material toward the lateral register plate 16a for lateral registration.
Consequently, an image printed on the second surface of the sheet material S is skewed or shifted in the lateral direction.
(2) If the sheet material S can not be adjusted to the lateral registry position as described above and is transported to the subsequent stage while being in the skewed state, this increases a possibility that the sheet material may jam somewhere in the course of transport because of a long distance by which the sheet material must be transported until being ejected.
The present invention has been made with the view of solving the above-mentioned problems in the related art, and its object is to provide a sheet material feeder and an image forming apparatus in which, even when a sheet of thick paper is transported through sheet material guide curving to a large extent, the sheet can be surely adjusted to a lateral registry position.
In accordance with these objects, there is provided a sheet material feeder with a sheet material guide means for guiding both surfaces of a sheet material while contacting the sheet material at a plurality of sheet contact positions, the sheet material guide means comprising a pair of curved sheet material guides, sheet material lateral position adjusting means for pushing both side edges of the sheet material in a direction perpendicular to a direction of sheet transport, thereby adjusting a widthwise position of the sheet material contained in the sheet material guide means, wherein the sheet material is easily movable in the sheet material guide means, while in contact with the sheet contact positions, when the widthwise position of the sheet material is adjusted.
Preferably, the sheet material feeder includes movable projection units providing the sheet contact positions of the pair of sheet material guide units and being able to move in the direction perpendicular to the direction of sheet transport.
Preferably, the movable projection units are provided movably in a direction toward one of the pair of sheet-material lateral position adjusting units which is positioned on the reference side.
Preferably, the movable projection units are provided movably in union with a base plate of the other of the pair of sheet-material lateral position adjusting units which is positioned on the side opposite to the reference side.
Preferably, rolls capable of rotating in the direction perpendicular to the direction of sheet transport are provided in the sheet contact positions of the pair of sheet material guide units.
Preferably, a material having a small frictional resistance is provided on surfaces of the sheet contact positions of the pair of sheet material guide units.
Preferably, the sheet material feeder includes a sheet material transport unit comprising a semicircular roller having a semicircular shape in section and a roll in pressure contact with the semicircular roller, and transporting the sheet material along the pair of sheet material guide units, and the roll is disposed in a state inclined in a direction to skew the sheet material toward one of the pair of sheet-material lateral position adjusting units which is positioned on the reference side.
Also, the present invention provides an image forming apparatus comprising the sheet material feeder set forth above, and an image forming unit for forming an image on a sheet material.
Preferably, the sheet material feeder is provided in a transport path along which a sheet material having an image formed thereon by the image forming unit is transported to the image forming unit again.
With the above features, even when a sheet of thick paper is transported through the sheet material guide means curving to a large extent, the sheet can be easily moved in the direction perpendicular to the direction of sheet transport and can be surely adjusted to a lateral registry position.